Act I

As Cavendish reveals, Doof is in fact Professor Time, the inventor of time travel fifteen years later, as he changes his name for branding purposes, but Doof doesn't believe a single word of this.

Meanwhile, at the Human Detention Center (formerly known as Lard World), Zack and Melissa, who have been captured, meet Buford and Baljeet, who were also captured, and doing some spying. Baljeet then recognizes Zack as the former lead singer of boy band the Lumberzacks. When Buford mentions that one of the pistachion guards were singing it, this gives Melissa an idea. So they distract the guards by singing the Lumberzacks' hit song, Chop Away at My Heart. While they sing, Melissa moves Martin toward a goat made of lard, hoping for Murphy's Law to go into effect. This takes a few seconds, but it finally occurs, causing the cage to fall apart, and the prisoners to escape. However, Martin, Brigette, and Sara are captured.

Back at Doofenshmirtz Evil Incorporated, now renamed "Professor Time's Building: Recently Renamed for Branding Purposes", Doof shows Milo and the others his time machine, the Time Traveler-inator, which, for some reason, resembles a taco. Right on cue, Agent P appears. As Doof reveals, the time machine runs on an undiscovered element called "time juice". As it turns out, the clocks that Dakota threw into the time stream were effected with time juice, and Milo happened to snag one, but it is back at his house. Unfortunately, Murphy's Law destroys Doof's time machine, so Milo goes back to the house with Diogee and Perry, while Doof rebuilds the time machine.

As Milo, Diogee, and Agent P go into the house, they are suddenly stopped by pistachions disguised as Milo's parents. As Milo evades them, he retrieves the clock, and they escape on Perry's jetpack.

Meanwhile, Melissa, Zack, Buford, and Baljeet are still running, when they come across Derek, disguised as Orton, explains his backstory. When he was in the time stream, he found himself in 1955. Ten years later, he infiltrated Balsawood Studios and took Orton's place as the creator and star of The Doctor Zone Files, which Melissa and the others are unfamiliar with in this timeline since Derek's version of the show was canceled after two episodes. As he takes off his mask, he discovers his face has shaped into Orton's. As he ponders his new face, the kids escape.

Milo returns to Doof's place with the clock, and now Doof has rebuilt the time machine to resemble a taco stand, just to stay on theme. Just as Doof begins to operate the time machine, some pistachions invade the lab. In the kerfuffle, Milo and Diogee stay behind, while Cavendish, Dakota, Doof, and Perry travel through time. Milo escapes from the pistachions using a rhythmic gymnastics routine.

In the time stream, it is discovered that there was only enough time juice to go into the time stream, but not enough to go out of the stream. So it appears they are destined to drift in the stream forever.

Meanwhile, Melissa, Zack, Buford, and Baljeet, while escaping from the pistachions, come to a dead end. They see a shadow coming their way, and they believe it to be another pistachion. But it turns out to be Candace, just casually strolling along while playing a Ducky Momo-themed augmented reality app on her phone. Suddenly, she and the rest of the kids are surrounded by pistachions, but they are saved by a giant tennis-themed robot, driven by none other than everyone's favorite boy inventors: Phineas and Ferb!

Act II

As the kids climb into Phineas and Ferb's robot, Zack and Melissa introduce themselves to Phineas and the gang, and vice versa. Phineas explains that the robot was initially going to be built with a counterpart so they could play tennis from opposite sides of Danville, but decided to use it to fight off the pistachions. Suddenly, the robot malfunctions, which could only mean one thing: Milo is back. As Ferb correctly surmises after the robot explodes, it is all Murphy's Law.

Back in the time stream, Doof, Dakota, Cavendish, and Perry are still drifting, and now wearing fake beards. Dakota is attempting to keep his sanity by listing all the animals in the zoo, since Cavendish forbade him from singing We're Going to the Zoo. And Doof and Perry play Go Fish with just one hot sauce packet. Suddenly, they notice the Dakota and Cavendish from the past throwing all the clocks into the stream. Dakota ties himself with rope and floats to get a clock.

They finally find themselves back in 1965, but at the Wyatt Burp orange soda factory, and a day later than their intended target, since Doof replaced the "date" function with a cup holder. As they approach the studio, they, unfortunately, discover they are too late to stop Derek from taking over. The pistachions from the past discover them and give chase.

Phineas, Milo, and the gangs arrive at one of Phineas's satellite workshops, which was used ever since the pistachions took over. As Murphy's Law continues to take its toll, Candace begins to doubt Milo's involvement. Baljeet suggests that they study and analyze Murphy's Law so that they could possibly use it to their advantage.

Back in 1965, Cavendish, Dakota, Orton and Perry wait for Doof to get his orange soda. After he returns, they make haste back onto the time machine.

In the present, Phineas, Milo, and the gangs begin to get to work, but as Milo's station falls apart, Candace starts to convince Milo to remove himself from the situation. As they depart, Zack and Melissa become suspicious about Candace being a pistachion, mainly because of her long neck, so they follow her and Milo.

Act III

Candace explains her pessimism to Milo but gradually changes her mind as she observes how Milo works around Murphy's Law. As Zack and Melissa attack her, they discover that she is not a pistachion after all.

In the time stream, Cavendish, Dakota, Orton, and Doof ponder where they should travel to next. As Doof opens a can of orange soda, the machine begins to malfunction. They land back in the present day on what appears to be a deserted island. Cavendish unleashes his rage at Doof's total incompetence, comparing him to even Dakota. Doof decides to separate from the group and gather his thoughts. He is soon joined by Orton, who sympathizes with him since he is under a lot of Pressure to be a great TV show creator, and when Doof picks up a ukulele, they musically soliloquize. After the song, they are surrounded by natives of the island.

Meanwhile, Cavendish and Dakota search for a way off the island, but then Dakota sees his own paradoxes, discovering that they are on the Island of Lost Dakotas, a fact that Dakota attempts to hide from Cavendish, but Cavendish soon finds out.

Back at the workshop, Candace completely changes her tune about Milo, and the gangs decide to study and analyze the effects of Murphy's Law, through a musical montage. As Baljeet explains something he calls "The Phineas and Ferb Effect (effect)", Milo, and Phineas and Ferb, each have different energy fields, with Milo giving off negatives ions, and Phineas and Ferb giving positive ions, and if Milo is surrounded by Phineas and Ferb, Murphy's Law can indeed be controlled. So in another montage, they build a giant robot resembling Milo called the Murphy's Law Suit.

Back on the island, Dakota finally tells Cavendish about how Cavendish died several times and that Dakota went back in time to save him, and ended up creating millions of paradoxes of himself and sent them on this island. The Dakota paradoxes recognize Cavendish, and Cavendish thanks them for what they have done. They let Doof and Orton, whom they have captured earlier, free. As Doof discovers, all the Dakotas run their economy by making their own off-brand orange sodas, which gives him an idea.

Just as Milo, Phineas, and Ferb are about to go into the robot, the workshop is swarmed by pistachions.

Act IV

As Milo, Phineas, Ferb, and their friends are taken into the Human Detention Center, Milo wonders why they haven't been replaced like the others. They are brought to a giant device known as a M.U.L.C.H. machine. As Derek broadcasts his diatribe to the world, he reveals that instead of replacing all of humankind with pistachions in disguise, they are going to transform all of the humans into pistachions like themselves, with M.U.L.C.H. standing for Machine Used for Literally Converting Humans To Plants (the last two letters, Derek insists, are silent). As the others suggest that they live together in harmony, the machine begins to suffer technical difficulties. Suddenly, Doof, Cavendish, Dakota, Orton, Perry, and the Dakota paradoxes come to the rescue with orange soda spray guns, which they fire at the pistachions. Isabella and the rest of the Fireside Girls arrive to free Milo, Phineas, and the others. A huge battle ensues. Derek, having had enough of the interlopers, sends out a giant pistachion to crush them, but the giant pistachion looks upon all the violence and reforms. During this heel-turn, the others escape.

Isabella and the Fireside Girls give Phineas, Milo, and Ferb the Murphy's Law Suit. The three go into the suit and use Murphy's Law as a weapon to crush many of the pistachions.

After a few minutes, Derek finally gets the M.U.L.C.H. machine working again. He begins by using it on Irving and Bradley, then moves on to Buford and Amanda. Just as the Murphy's Law Suit is about to aim at the machine, a few pistachions sabotage it. The suit aims at the saboteurs but causes further damage to the power pack. As Milo, Phineas, and Ferb survey the damage, they get an idea to use the acid from lemons in Milo's backpack as a battery. When Milo produces too few lemons, he suggests using a radioactive watermelon instead.

Meanwhile, even Linda , Lawrence, and Milo's parents have been transformed. The reactivated super suit destroys the machine, but it is too late, as a majority of the Phineas and Milo characters have been transformed. The transformed pistachions attempt to destroy the super suit, but Ferb disconnects the right arm before they can. Milo, Phineas, and Ferb reunite with the remaining survivors, Dakota, Cavendish, Orton, and Doof. They give the transformed pistachions chase on the rollercoaster. As they reach a dead end, Doof has an epiphany, which Orton writes down for his memoirs. Suddenly, Professor Time himself emerges from the future at that exact moment, having read Orton's memoirs. Professor Time gives Orton his Doctor Zone hat, and he joins him in his time machine. Before they leave, Milo gives Orton the letter that he left for Zack and Melissa to find.

In 1955, a young Derek devises his evil scheme but is stopped by Orton and Professor Time, who kill him with the orange soda.

Back in the present, Derek, having been defeated almost 50 years earlier, begins to fade away from existence. The rest of the pistachions start to disappear as well, and all of the transformed pistachions return to normal, except for Bradley, whose left arm is still part-plant. Even the Human Detention Center is returned to its pristine condition as Lard World. Dakota and Cavendish ponder the number of violations Mr. Block will hold them accountable for. Just as Milo shows his relief that everything is back to normal, Murphy's Law goes into effect and destroys Doof's building.

At Milo's house, Milo, Zack, Melissa, Phineas and the gang watch an episode of The Doctor Zone Files. As Phineas and the gang leave, Milo invites them to cross over any time, since they live on the other side of town. Right after they depart, Doof, who is now crashing with Milo, asks where they keep the toilet paper.

A Spanish promo was released in June but aired only once on Disney XD Spain.

A first English promo was released on August 9th, 2018, and announced that the crossover would air in the U.S.A in January 2019.

On August 5, 2018, the crossover was pre-screened to special (non-general) audiences in Tokyo to showcase the episode and its focus point on the series and they invited 120 people coming from 60 groups. It was screened twice, with the first screening which could be attended in the middle of the day for anyone 18 years or older in addition to those 6 years or older with a parent or guardian and the second screening being attended hours later for only those who are 18 years or older.[11]

The Fireside Girls are wearing recoloured outfits similar to the Firestorm Girls, their second dimension counterparts who are also fighting for their Danville's freedom.

Vincent Martella is credited for playing Bradley and then Phineas, even though Phineas has more lines.

It could be a rule that the character from Milo Murphy's Law that a person voices has to be credited before a crossover character from Phineas and Ferb, like Diogee being listed before Perry when Dee Bradley Baker voices both of them.

Errors

Near the end of "Fungus Among Us", Dakota tells Cavendish that Professor Time won't invent time travel for another twelve years. However, in the beginning of this episode, Cavendish tells Doofenshmirtz that he'll invent time travel in fifteen years.

Milo tells Orton Mahlson that in 50 years he has to give the letter to Melissa and Zack on last week. That's impossible since in Fungus Among Us Dakota says that they are in Milo's time but two weeks later. Therefore, events of Missing Milo should've happened 3 weeks ago from now.

Doofenshmirtz isn't the first to invent time travel, since Phineas and Ferb fixed the time machine in the museum in the episodes "It's About Time!" and " Phineas and Ferb's Quantum Boogaloo". But Doof probably wasn't aware of that, unless he later went back in time to help Xavier Onassis.

When Phineas, Ferb and Milo strap on their seatbelts in the robot, Ferb's larger eye is in front of his smaller one.

Doof's building has the sign changed to "Professor Time's Building, Recently renamed for branding purposes" (and is later destroyed). However, in " Act Your Age", which takes place 10 years in the future, Doof's building is still Doofenshmirtz Evil Incorporated, except the word "Evil" has been removed from the sign. However, it is likely that Doofenshmirtz just decided to rename his sign with its old name (minus the word "Evil").

This crossover has several inconsistencies with the original Phineas and Ferb continuity:

The Phineas and Ferb Effect of everything going right for Phineas and Ferb does not add up with their Sun-Beater getting damaged along the way in "Phineas and Ferb Summer Belongs to you!", getting their rubberization machine broken by Doof-zombies and getting turned into them before Isabella in " Night of the Living Pharmacists", or being sucked through a time rift in "Last Day of Summer" in which the conflicts came from things going wrong for the boys. Additionally, the Phineas and Ferb Effect as a concept ruins any tension that "Last Day of Summer" originally had from Candace's do-overs to bust them.

"Candace Loses Her Head" and "The Best Lazy Day Ever" indicate that Phineas and Ferb takes place in 2008, and technology in the show is usually dated back to that time as it goes on. "Battle of the Bands" and "The Llama Incident" indicate that Milo Murphy's Law takes place in 2017, which is after 2015 when The Doctor Zone Files would be at least 50 years old. This could mean that the future storyline of "Act Your Age" may not even exist in Milo Murphy's Law continuity since every Phineas and Ferb character is the same age almost 10 years after 2008 in Milo Murphy's Law.

Dr. Doofenshmirtz had other jobs after he gave up evil as Vanessa's science teacher in "Doof 101" and "The O.W.C.A Files". In Milo Murphy's Law, when he moves into Milo's house after DEI is destroyed, he appears to be jobless and it is never explained how he may have lost those jobs. It is possible, however, that either of them hasn't happened yet: in the case of "The O.W.C.A. Files", it was ever established when that episode took place.

Allusions

Rambo - Isabella's outfit in this episode is a spoof of Rambo's look.

The Lord of the Rings: The Return of the King- The speech Doof makes when they were corned is similar to this one.

Pokémon GO - The game that Candace plays on her phone, Ducky Mo-Go, is a spoof of Pokémon GO, which became a really popular game in 2016.

Futurama - One of the pistachions calls Phineas and his friends “meatbags”, similar to Bender, one of the main characters of the animated sitcom.

In God, We Trust - The coin Doofenshmirtz has during the song “Pressure” has “In Doof We Trust.” This is a reference to the motto to the United States of America and Florida.

You Must Be This Tall To Ride - There is a sign during the song “Pressure” that says “you must be this tall to invent time travel.” This is a trope in popular culture and a minimum height restriction in real life.

Uptown Funk - The song "How Do I Do It?" has an instrumental vibe similar to this Mark Ronson/Bruno Mars hit song. Also, while Milo, Melissa, Zack, Phineas and Ferb dance to the song, they wear clothes and dance in a style, which are both similar to the music video of "Uptown Funk".

The Excorist - Doof says "The power of orange soda compels you!" three times, which is a reference to Father Merrin and Father Karras' repeated line "The power of Christ comples you!" from the 1973 film.

The Dab - During the cover of the song "Chop Away at My Heart", Baljeet does the Dab.

Saving Private Ryan - A parody of the opening of "Saving Private Ryan" is shown during the battle between humans and pistachions, when a giant pistachion is sent out by Derek to crush the humans but looks upon all the violence and reforms. We see pistachions getting injured, killed, and attacked by the humans with orange soda. We also see some Dakota clones hiding in fear.

iCarly - Derek counting backwards to one but not saying one is similar to Freddie Benson, one of the main characters from the live-action Nickelodeon Sitcom.

Kamen Rider Build - Derek planning to turn people into plant monsters is similar to Faust, one of the antagonists of the 2017-2018 tokusatsu Superhero series, by turning people into monsters called Smash.

Super Sentai/Power Rangers - Milo, Phineas and Ferb using a robot to defeat the Pistachions is similar to the protagonists using a giant robot to defeat the monsters they fight.

Adventure Time - Doofenshmirtz staying at Milo’s house when his home is destroyed is similar to the Ice King staying with Finn and Jake when the Ice Kingdom is destroyed after the events of the episode “Frost and Fire”.

Delays

The crossover has been delayed a few times:

Dan Povenmire originally said he thought it would air in April 2018, until Chrissie Fit said on Twitter that she thought it was now June.

Dan Povenmire said it was coming "Sometime in 2018", and the fanbase believed it to be a delay, even though it was not.

According to Al Yankovic, it would air in August of that year on Disney Channel.[12]

On June 1, 2018, Povenmire said on Twitter that the crossover would come "later this year".[13]

On July 13, 2018, Disney Channel revealed that the crossover would not air in America in August.[14]

On August 9, 2018, it was confirmed that the crossover would air on January 2019 on Disney Channel.[15]